A solid-state image-sensing device is classified into a CIS type device and a charge-coupled device (CCD). Recently, the CIS type solid-state image-sensing device has replaced the CCD type solid-state image-sensing device and used in a variety of fields because the CIS type solid-state image-sensing device requires a operating voltage and consumption power lower than those of the CCD type solid-state image-sensing device and it is fabricated using a standard CMOS technology and easily integrated.
The CIS type solid-state image-sensing device is mounted in a cellular phone camera and a digital still camera, captures an image, converts the captured image into an electric signal and transmits the electric signal to a digital signal processor. The digital signal processor processes color image data RGB output from the solid-state image-sensing device to drive a display device such as a liquid crystal display (LCD).
In a system employing the CIS type solid-state image-sensing device, a sub-sampling mode of the solid-state image-sensing device outputs a video signal with a reduced vertical resolution. The sub-sampling mode supports a high frame rate in steps of requiring no high-resolution display, which include a moving picture displaying step, a preview step of confirming an image before capturing the image, and an automatic focusing step.
FIG. 1 is a block diagram of a conventional CIS type solid-state image-sensing device 100. Referring to FIG. 1, the conventional CIS type solid-state image-sensing device 100 includes an active pixel sensor (APS) array 110, a row driver 120 and an analog-digital conversion unit 130. The row driver 120 receives a control signal from a row decoder (not shown) and the analog-digital conversion unit 130 receives a control signal from a column decoder (not shown). The solid-state image-sensing device 100 further includes a controller (not shown) that generates timing control signals and addressing signals for outputting a selected and sensed video signal of each pixel. In the solid-state image-sensing device 100, in general, a color filter is arranged on each of pixels constructing the APS array 110 such that only light of a specific color is input to each pixel. To construct color signals, at least three kinks of color filters are arranged on the APS array 110. A general color filter array has Bayer pattern in which two red and green color patterns are arranged in one row and two green and blue color patterns are arranged in another row. Here, the green color pattern closed related to a luminance signal is arranged in all rows and the red and blue color patterns are alternately arranged in rows to improve luminance resolution. A CIS having more than one million pixels is applied to a digital still camera in order to improve resolution.
In the CIS type solid-state image-sensing device 100, the APS array 110 senses light using a photodiode and converts the sensed light into electric signals to generate video signals. The video signals output from the APS array 110 include red (R), green (G) and blue (B) analog signals. The analog-digital conversion unit 130 receives the analog video signals output from the APS array 110 and converts the analog video signals into digital signals.
In the conventional CIS type solid-state image-sensing device 100 shown in FIG. 1, the analog-digital conversion unit 130 converts a video signal sensed by the photodiode into a digital signal using a correlated double sampling (CDS) method, which is disclosed in U.S. Pat. No. 5,982,318 and No. 6,067,113. CDS analog-digital conversion is divided into two steps of receiving a reset signal from the APS array 110 and then receiving a video signal sensed by the photodiode to convert the video signal into a digital signal. Whenever the photodiode newly senses light at a predetermined period, the APS array 110 outputs the reset signal to the analog-digital conversion unit 130 before the photodiode outputs a newly sensed video signal to the analog-digital conversion unit 130. The analog-digital conversion unit 110 receives the reset signal to be reset, and then converts the video signal received from the photodiode into a digital signal. The digital signal is output to a digital signal processor and interpolated. The digital signal processor generates a driving signal suitable for resolution of a display device such as LCD to drive the display device.
When the CIS solid-state image-sensing device captures a still image, video signals of all pixels, sensed by photodiodes of the APS array 110 are output. In the sub-sampling mode, however, vertical resolution is reduced and video signals are output. In case of a CIS type solid-state image-sensing device having an APS array with super extended graphic adapter (SXGA) resolution, for example, the solid-state image-sensing device outputs SXGA-grade video signals when it photographs a still image. However, the solid-state image-sensing device outputs video graphic adapter (VGA)-grade video signals in sub-sampling mode operations including moving picture display, preview and automatic focus. For reference, the number of pixels of SXGA resolution is 1280*1024 and the number of pixels of VGA resolution is 640*480. Even a CIS type solid-state image-sensing device having an APS array with ultra extended graphics adapter (UXGA) resolution outputs video signals with less than VGA-grade resolution in the sub-sampling mode to reduce the quantity of processed data. For reference, the number of pixels of UXGA resolution is 1600*1200.
In the sub-sampling mode of the conventional CIS type solid-state image-sensing device 100, only video signals of a specific row and column are output to the analog-digital conversion unit 130 to reduce vertical resolution. To decrease the SXGA resolution to the VGA resolution, for instance, only a single data corresponding to the intersection of one row and column is selected from pixel data items corresponding to two rows and two columns and other data items are removed such that resolution is reduced by half. When only data corresponding to one row and column is selected from data items corresponding to many rows and columns, the resolution can be further reduced and thus the quantity of processed data can be further decreased.
However, there exits data that is not used but discarded in the sub-sampling mode of the conventional CIS type solid-state image-sensing device 100. This causes aliasing noise that oblique lines on a display are not smoothly connected but shown zigzag. To remove the aliasing noise, a method of averaging video signals in a predetermined range and outputting the averaged video signal has been proposed. The video signals averaging method includes a method of analog-averaging video signals in a predetermined range before video signals sensed by pixels are output to the analog-digital conversion unit 130, and a method of averaging corresponding digital signals output from the analog-digital conversion unit 130. However, digital averaging requires a large-capacity memory so that a chip area and power consumption are increased. Furthermore, to analog-average video signals sensed by pixels in the structure shown in FIG. 1, a single column requires two large capacitors for a reset signal and a video signal to result in an increase in the chip area. Accordingly, the solid-state image-sensing device is difficult to apply to small-size mobile apparatuses.